1<?xml version="1.0" encoding="UTF-8"?> 2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" 3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []> 4 5<book id="KernelCryptoAPI"> 6 <bookinfo> 7 <title>Linux Kernel Crypto API</title> 8 9 <authorgroup> 10 <author> 11 <firstname>Stephan</firstname> 12 <surname>Mueller</surname> 13 <affiliation> 14 <address> 15 <email>smueller@chronox.de</email> 16 </address> 17 </affiliation> 18 </author> 19 <author> 20 <firstname>Marek</firstname> 21 <surname>Vasut</surname> 22 <affiliation> 23 <address> 24 <email>marek@denx.de</email> 25 </address> 26 </affiliation> 27 </author> 28 </authorgroup> 29 30 <copyright> 31 <year>2014</year> 32 <holder>Stephan Mueller</holder> 33 </copyright> 34 35 36 <legalnotice> 37 <para> 38 This documentation is free software; you can redistribute 39 it and/or modify it under the terms of the GNU General Public 40 License as published by the Free Software Foundation; either 41 version 2 of the License, or (at your option) any later 42 version. 43 </para> 44 45 <para> 46 This program is distributed in the hope that it will be 47 useful, but WITHOUT ANY WARRANTY; without even the implied 48 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 49 See the GNU General Public License for more details. 50 </para> 51 52 <para> 53 You should have received a copy of the GNU General Public 54 License along with this program; if not, write to the Free 55 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, 56 MA 02111-1307 USA 57 </para> 58 59 <para> 60 For more details see the file COPYING in the source 61 distribution of Linux. 62 </para> 63 </legalnotice> 64 </bookinfo> 65 66 <toc></toc> 67 68 <chapter id="Intro"> 69 <title>Kernel Crypto API Interface Specification</title> 70 71 <sect1><title>Introduction</title> 72 73 <para> 74 The kernel crypto API offers a rich set of cryptographic ciphers as 75 well as other data transformation mechanisms and methods to invoke 76 these. This document contains a description of the API and provides 77 example code. 78 </para> 79 80 <para> 81 To understand and properly use the kernel crypto API a brief 82 explanation of its structure is given. Based on the architecture, 83 the API can be separated into different components. Following the 84 architecture specification, hints to developers of ciphers are 85 provided. Pointers to the API function call documentation are 86 given at the end. 87 </para> 88 89 <para> 90 The kernel crypto API refers to all algorithms as "transformations". 91 Therefore, a cipher handle variable usually has the name "tfm". 92 Besides cryptographic operations, the kernel crypto API also knows 93 compression transformations and handles them the same way as ciphers. 94 </para> 95 96 <para> 97 The kernel crypto API serves the following entity types: 98 99 <itemizedlist> 100 <listitem> 101 <para>consumers requesting cryptographic services</para> 102 </listitem> 103 <listitem> 104 <para>data transformation implementations (typically ciphers) 105 that can be called by consumers using the kernel crypto 106 API</para> 107 </listitem> 108 </itemizedlist> 109 </para> 110 111 <para> 112 This specification is intended for consumers of the kernel crypto 113 API as well as for developers implementing ciphers. This API 114 specification, however, does not discuss all API calls available 115 to data transformation implementations (i.e. implementations of 116 ciphers and other transformations (such as CRC or even compression 117 algorithms) that can register with the kernel crypto API). 118 </para> 119 120 <para> 121 Note: The terms "transformation" and cipher algorithm are used 122 interchangeably. 123 </para> 124 </sect1> 125 126 <sect1><title>Terminology</title> 127 <para> 128 The transformation implementation is an actual code or interface 129 to hardware which implements a certain transformation with precisely 130 defined behavior. 131 </para> 132 133 <para> 134 The transformation object (TFM) is an instance of a transformation 135 implementation. There can be multiple transformation objects 136 associated with a single transformation implementation. Each of 137 those transformation objects is held by a crypto API consumer or 138 another transformation. Transformation object is allocated when a 139 crypto API consumer requests a transformation implementation. 140 The consumer is then provided with a structure, which contains 141 a transformation object (TFM). 142 </para> 143 144 <para> 145 The structure that contains transformation objects may also be 146 referred to as a "cipher handle". Such a cipher handle is always 147 subject to the following phases that are reflected in the API calls 148 applicable to such a cipher handle: 149 </para> 150 151 <orderedlist> 152 <listitem> 153 <para>Initialization of a cipher handle.</para> 154 </listitem> 155 <listitem> 156 <para>Execution of all intended cipher operations applicable 157 for the handle where the cipher handle must be furnished to 158 every API call.</para> 159 </listitem> 160 <listitem> 161 <para>Destruction of a cipher handle.</para> 162 </listitem> 163 </orderedlist> 164 165 <para> 166 When using the initialization API calls, a cipher handle is 167 created and returned to the consumer. Therefore, please refer 168 to all initialization API calls that refer to the data 169 structure type a consumer is expected to receive and subsequently 170 to use. The initialization API calls have all the same naming 171 conventions of crypto_alloc_*. 172 </para> 173 174 <para> 175 The transformation context is private data associated with 176 the transformation object. 177 </para> 178 </sect1> 179 </chapter> 180 181 <chapter id="Architecture"><title>Kernel Crypto API Architecture</title> 182 <sect1><title>Cipher algorithm types</title> 183 <para> 184 The kernel crypto API provides different API calls for the 185 following cipher types: 186 187 <itemizedlist> 188 <listitem><para>Symmetric ciphers</para></listitem> 189 <listitem><para>AEAD ciphers</para></listitem> 190 <listitem><para>Message digest, including keyed message digest</para></listitem> 191 <listitem><para>Random number generation</para></listitem> 192 <listitem><para>User space interface</para></listitem> 193 </itemizedlist> 194 </para> 195 </sect1> 196 197 <sect1><title>Ciphers And Templates</title> 198 <para> 199 The kernel crypto API provides implementations of single block 200 ciphers and message digests. In addition, the kernel crypto API 201 provides numerous "templates" that can be used in conjunction 202 with the single block ciphers and message digests. Templates 203 include all types of block chaining mode, the HMAC mechanism, etc. 204 </para> 205 206 <para> 207 Single block ciphers and message digests can either be directly 208 used by a caller or invoked together with a template to form 209 multi-block ciphers or keyed message digests. 210 </para> 211 212 <para> 213 A single block cipher may even be called with multiple templates. 214 However, templates cannot be used without a single cipher. 215 </para> 216 217 <para> 218 See /proc/crypto and search for "name". For example: 219 220 <itemizedlist> 221 <listitem><para>aes</para></listitem> 222 <listitem><para>ecb(aes)</para></listitem> 223 <listitem><para>cmac(aes)</para></listitem> 224 <listitem><para>ccm(aes)</para></listitem> 225 <listitem><para>rfc4106(gcm(aes))</para></listitem> 226 <listitem><para>sha1</para></listitem> 227 <listitem><para>hmac(sha1)</para></listitem> 228 <listitem><para>authenc(hmac(sha1),cbc(aes))</para></listitem> 229 </itemizedlist> 230 </para> 231 232 <para> 233 In these examples, "aes" and "sha1" are the ciphers and all 234 others are the templates. 235 </para> 236 </sect1> 237 238 <sect1><title>Synchronous And Asynchronous Operation</title> 239 <para> 240 The kernel crypto API provides synchronous and asynchronous 241 API operations. 242 </para> 243 244 <para> 245 When using the synchronous API operation, the caller invokes 246 a cipher operation which is performed synchronously by the 247 kernel crypto API. That means, the caller waits until the 248 cipher operation completes. Therefore, the kernel crypto API 249 calls work like regular function calls. For synchronous 250 operation, the set of API calls is small and conceptually 251 similar to any other crypto library. 252 </para> 253 254 <para> 255 Asynchronous operation is provided by the kernel crypto API 256 which implies that the invocation of a cipher operation will 257 complete almost instantly. That invocation triggers the 258 cipher operation but it does not signal its completion. Before 259 invoking a cipher operation, the caller must provide a callback 260 function the kernel crypto API can invoke to signal the 261 completion of the cipher operation. Furthermore, the caller 262 must ensure it can handle such asynchronous events by applying 263 appropriate locking around its data. The kernel crypto API 264 does not perform any special serialization operation to protect 265 the caller's data integrity. 266 </para> 267 </sect1> 268 269 <sect1><title>Crypto API Cipher References And Priority</title> 270 <para> 271 A cipher is referenced by the caller with a string. That string 272 has the following semantics: 273 274 <programlisting> 275 template(single block cipher) 276 </programlisting> 277 278 where "template" and "single block cipher" is the aforementioned 279 template and single block cipher, respectively. If applicable, 280 additional templates may enclose other templates, such as 281 282 <programlisting> 283 template1(template2(single block cipher))) 284 </programlisting> 285 </para> 286 287 <para> 288 The kernel crypto API may provide multiple implementations of a 289 template or a single block cipher. For example, AES on newer 290 Intel hardware has the following implementations: AES-NI, 291 assembler implementation, or straight C. Now, when using the 292 string "aes" with the kernel crypto API, which cipher 293 implementation is used? The answer to that question is the 294 priority number assigned to each cipher implementation by the 295 kernel crypto API. When a caller uses the string to refer to a 296 cipher during initialization of a cipher handle, the kernel 297 crypto API looks up all implementations providing an 298 implementation with that name and selects the implementation 299 with the highest priority. 300 </para> 301 302 <para> 303 Now, a caller may have the need to refer to a specific cipher 304 implementation and thus does not want to rely on the 305 priority-based selection. To accommodate this scenario, the 306 kernel crypto API allows the cipher implementation to register 307 a unique name in addition to common names. When using that 308 unique name, a caller is therefore always sure to refer to 309 the intended cipher implementation. 310 </para> 311 312 <para> 313 The list of available ciphers is given in /proc/crypto. However, 314 that list does not specify all possible permutations of 315 templates and ciphers. Each block listed in /proc/crypto may 316 contain the following information -- if one of the components 317 listed as follows are not applicable to a cipher, it is not 318 displayed: 319 </para> 320 321 <itemizedlist> 322 <listitem> 323 <para>name: the generic name of the cipher that is subject 324 to the priority-based selection -- this name can be used by 325 the cipher allocation API calls (all names listed above are 326 examples for such generic names)</para> 327 </listitem> 328 <listitem> 329 <para>driver: the unique name of the cipher -- this name can 330 be used by the cipher allocation API calls</para> 331 </listitem> 332 <listitem> 333 <para>module: the kernel module providing the cipher 334 implementation (or "kernel" for statically linked ciphers)</para> 335 </listitem> 336 <listitem> 337 <para>priority: the priority value of the cipher implementation</para> 338 </listitem> 339 <listitem> 340 <para>refcnt: the reference count of the respective cipher 341 (i.e. the number of current consumers of this cipher)</para> 342 </listitem> 343 <listitem> 344 <para>selftest: specification whether the self test for the 345 cipher passed</para> 346 </listitem> 347 <listitem> 348 <para>type: 349 <itemizedlist> 350 <listitem> 351 <para>skcipher for symmetric key ciphers</para> 352 </listitem> 353 <listitem> 354 <para>cipher for single block ciphers that may be used with 355 an additional template</para> 356 </listitem> 357 <listitem> 358 <para>shash for synchronous message digest</para> 359 </listitem> 360 <listitem> 361 <para>ahash for asynchronous message digest</para> 362 </listitem> 363 <listitem> 364 <para>aead for AEAD cipher type</para> 365 </listitem> 366 <listitem> 367 <para>compression for compression type transformations</para> 368 </listitem> 369 <listitem> 370 <para>rng for random number generator</para> 371 </listitem> 372 <listitem> 373 <para>givcipher for cipher with associated IV generator 374 (see the geniv entry below for the specification of the 375 IV generator type used by the cipher implementation)</para> 376 </listitem> 377 </itemizedlist> 378 </para> 379 </listitem> 380 <listitem> 381 <para>blocksize: blocksize of cipher in bytes</para> 382 </listitem> 383 <listitem> 384 <para>keysize: key size in bytes</para> 385 </listitem> 386 <listitem> 387 <para>ivsize: IV size in bytes</para> 388 </listitem> 389 <listitem> 390 <para>seedsize: required size of seed data for random number 391 generator</para> 392 </listitem> 393 <listitem> 394 <para>digestsize: output size of the message digest</para> 395 </listitem> 396 <listitem> 397 <para>geniv: IV generation type: 398 <itemizedlist> 399 <listitem> 400 <para>eseqiv for encrypted sequence number based IV 401 generation</para> 402 </listitem> 403 <listitem> 404 <para>seqiv for sequence number based IV generation</para> 405 </listitem> 406 <listitem> 407 <para>chainiv for chain iv generation</para> 408 </listitem> 409 <listitem> 410 <para><builtin> is a marker that the cipher implements 411 IV generation and handling as it is specific to the given 412 cipher</para> 413 </listitem> 414 </itemizedlist> 415 </para> 416 </listitem> 417 </itemizedlist> 418 </sect1> 419 420 <sect1><title>Key Sizes</title> 421 <para> 422 When allocating a cipher handle, the caller only specifies the 423 cipher type. Symmetric ciphers, however, typically support 424 multiple key sizes (e.g. AES-128 vs. AES-192 vs. AES-256). 425 These key sizes are determined with the length of the provided 426 key. Thus, the kernel crypto API does not provide a separate 427 way to select the particular symmetric cipher key size. 428 </para> 429 </sect1> 430 431 <sect1><title>Cipher Allocation Type And Masks</title> 432 <para> 433 The different cipher handle allocation functions allow the 434 specification of a type and mask flag. Both parameters have 435 the following meaning (and are therefore not covered in the 436 subsequent sections). 437 </para> 438 439 <para> 440 The type flag specifies the type of the cipher algorithm. 441 The caller usually provides a 0 when the caller wants the 442 default handling. Otherwise, the caller may provide the 443 following selections which match the aforementioned cipher 444 types: 445 </para> 446 447 <itemizedlist> 448 <listitem> 449 <para>CRYPTO_ALG_TYPE_CIPHER Single block cipher</para> 450 </listitem> 451 <listitem> 452 <para>CRYPTO_ALG_TYPE_COMPRESS Compression</para> 453 </listitem> 454 <listitem> 455 <para>CRYPTO_ALG_TYPE_AEAD Authenticated Encryption with 456 Associated Data (MAC)</para> 457 </listitem> 458 <listitem> 459 <para>CRYPTO_ALG_TYPE_BLKCIPHER Synchronous multi-block cipher</para> 460 </listitem> 461 <listitem> 462 <para>CRYPTO_ALG_TYPE_ABLKCIPHER Asynchronous multi-block cipher</para> 463 </listitem> 464 <listitem> 465 <para>CRYPTO_ALG_TYPE_GIVCIPHER Asynchronous multi-block 466 cipher packed together with an IV generator (see geniv field 467 in the /proc/crypto listing for the known IV generators)</para> 468 </listitem> 469 <listitem> 470 <para>CRYPTO_ALG_TYPE_DIGEST Raw message digest</para> 471 </listitem> 472 <listitem> 473 <para>CRYPTO_ALG_TYPE_HASH Alias for CRYPTO_ALG_TYPE_DIGEST</para> 474 </listitem> 475 <listitem> 476 <para>CRYPTO_ALG_TYPE_SHASH Synchronous multi-block hash</para> 477 </listitem> 478 <listitem> 479 <para>CRYPTO_ALG_TYPE_AHASH Asynchronous multi-block hash</para> 480 </listitem> 481 <listitem> 482 <para>CRYPTO_ALG_TYPE_RNG Random Number Generation</para> 483 </listitem> 484 <listitem> 485 <para>CRYPTO_ALG_TYPE_AKCIPHER Asymmetric cipher</para> 486 </listitem> 487 <listitem> 488 <para>CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of 489 CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression / 490 decompression instead of performing the operation on one 491 segment only. CRYPTO_ALG_TYPE_PCOMPRESS is intended to replace 492 CRYPTO_ALG_TYPE_COMPRESS once existing consumers are converted.</para> 493 </listitem> 494 </itemizedlist> 495 496 <para> 497 The mask flag restricts the type of cipher. The only allowed 498 flag is CRYPTO_ALG_ASYNC to restrict the cipher lookup function 499 to asynchronous ciphers. Usually, a caller provides a 0 for the 500 mask flag. 501 </para> 502 503 <para> 504 When the caller provides a mask and type specification, the 505 caller limits the search the kernel crypto API can perform for 506 a suitable cipher implementation for the given cipher name. 507 That means, even when a caller uses a cipher name that exists 508 during its initialization call, the kernel crypto API may not 509 select it due to the used type and mask field. 510 </para> 511 </sect1> 512 513 <sect1><title>Internal Structure of Kernel Crypto API</title> 514 515 <para> 516 The kernel crypto API has an internal structure where a cipher 517 implementation may use many layers and indirections. This section 518 shall help to clarify how the kernel crypto API uses 519 various components to implement the complete cipher. 520 </para> 521 522 <para> 523 The following subsections explain the internal structure based 524 on existing cipher implementations. The first section addresses 525 the most complex scenario where all other scenarios form a logical 526 subset. 527 </para> 528 529 <sect2><title>Generic AEAD Cipher Structure</title> 530 531 <para> 532 The following ASCII art decomposes the kernel crypto API layers 533 when using the AEAD cipher with the automated IV generation. The 534 shown example is used by the IPSEC layer. 535 </para> 536 537 <para> 538 For other use cases of AEAD ciphers, the ASCII art applies as 539 well, but the caller may not use the AEAD cipher with a separate 540 IV generator. In this case, the caller must generate the IV. 541 </para> 542 543 <para> 544 The depicted example decomposes the AEAD cipher of GCM(AES) based 545 on the generic C implementations (gcm.c, aes-generic.c, ctr.c, 546 ghash-generic.c, seqiv.c). The generic implementation serves as an 547 example showing the complete logic of the kernel crypto API. 548 </para> 549 550 <para> 551 It is possible that some streamlined cipher implementations (like 552 AES-NI) provide implementations merging aspects which in the view 553 of the kernel crypto API cannot be decomposed into layers any more. 554 In case of the AES-NI implementation, the CTR mode, the GHASH 555 implementation and the AES cipher are all merged into one cipher 556 implementation registered with the kernel crypto API. In this case, 557 the concept described by the following ASCII art applies too. However, 558 the decomposition of GCM into the individual sub-components 559 by the kernel crypto API is not done any more. 560 </para> 561 562 <para> 563 Each block in the following ASCII art is an independent cipher 564 instance obtained from the kernel crypto API. Each block 565 is accessed by the caller or by other blocks using the API functions 566 defined by the kernel crypto API for the cipher implementation type. 567 </para> 568 569 <para> 570 The blocks below indicate the cipher type as well as the specific 571 logic implemented in the cipher. 572 </para> 573 574 <para> 575 The ASCII art picture also indicates the call structure, i.e. who 576 calls which component. The arrows point to the invoked block 577 where the caller uses the API applicable to the cipher type 578 specified for the block. 579 </para> 580 581 <programlisting> 582<![CDATA[ 583kernel crypto API | IPSEC Layer 584 | 585+-----------+ | 586| | (1) 587| aead | <----------------------------------- esp_output 588| (seqiv) | ---+ 589+-----------+ | 590 | (2) 591+-----------+ | 592| | <--+ (2) 593| aead | <----------------------------------- esp_input 594| (gcm) | ------------+ 595+-----------+ | 596 | (3) | (5) 597 v v 598+-----------+ +-----------+ 599| | | | 600| skcipher | | ahash | 601| (ctr) | ---+ | (ghash) | 602+-----------+ | +-----------+ 603 | 604+-----------+ | (4) 605| | <--+ 606| cipher | 607| (aes) | 608+-----------+ 609]]> 610 </programlisting> 611 612 <para> 613 The following call sequence is applicable when the IPSEC layer 614 triggers an encryption operation with the esp_output function. During 615 configuration, the administrator set up the use of rfc4106(gcm(aes)) as 616 the cipher for ESP. The following call sequence is now depicted in the 617 ASCII art above: 618 </para> 619 620 <orderedlist> 621 <listitem> 622 <para> 623 esp_output() invokes crypto_aead_encrypt() to trigger an encryption 624 operation of the AEAD cipher with IV generator. 625 </para> 626 627 <para> 628 In case of GCM, the SEQIV implementation is registered as GIVCIPHER 629 in crypto_rfc4106_alloc(). 630 </para> 631 632 <para> 633 The SEQIV performs its operation to generate an IV where the core 634 function is seqiv_geniv(). 635 </para> 636 </listitem> 637 638 <listitem> 639 <para> 640 Now, SEQIV uses the AEAD API function calls to invoke the associated 641 AEAD cipher. In our case, during the instantiation of SEQIV, the 642 cipher handle for GCM is provided to SEQIV. This means that SEQIV 643 invokes AEAD cipher operations with the GCM cipher handle. 644 </para> 645 646 <para> 647 During instantiation of the GCM handle, the CTR(AES) and GHASH 648 ciphers are instantiated. The cipher handles for CTR(AES) and GHASH 649 are retained for later use. 650 </para> 651 652 <para> 653 The GCM implementation is responsible to invoke the CTR mode AES and 654 the GHASH cipher in the right manner to implement the GCM 655 specification. 656 </para> 657 </listitem> 658 659 <listitem> 660 <para> 661 The GCM AEAD cipher type implementation now invokes the SKCIPHER API 662 with the instantiated CTR(AES) cipher handle. 663 </para> 664 665 <para> 666 During instantiation of the CTR(AES) cipher, the CIPHER type 667 implementation of AES is instantiated. The cipher handle for AES is 668 retained. 669 </para> 670 671 <para> 672 That means that the SKCIPHER implementation of CTR(AES) only 673 implements the CTR block chaining mode. After performing the block 674 chaining operation, the CIPHER implementation of AES is invoked. 675 </para> 676 </listitem> 677 678 <listitem> 679 <para> 680 The SKCIPHER of CTR(AES) now invokes the CIPHER API with the AES 681 cipher handle to encrypt one block. 682 </para> 683 </listitem> 684 685 <listitem> 686 <para> 687 The GCM AEAD implementation also invokes the GHASH cipher 688 implementation via the AHASH API. 689 </para> 690 </listitem> 691 </orderedlist> 692 693 <para> 694 When the IPSEC layer triggers the esp_input() function, the same call 695 sequence is followed with the only difference that the operation starts 696 with step (2). 697 </para> 698 </sect2> 699 700 <sect2><title>Generic Block Cipher Structure</title> 701 <para> 702 Generic block ciphers follow the same concept as depicted with the ASCII 703 art picture above. 704 </para> 705 706 <para> 707 For example, CBC(AES) is implemented with cbc.c, and aes-generic.c. The 708 ASCII art picture above applies as well with the difference that only 709 step (4) is used and the SKCIPHER block chaining mode is CBC. 710 </para> 711 </sect2> 712 713 <sect2><title>Generic Keyed Message Digest Structure</title> 714 <para> 715 Keyed message digest implementations again follow the same concept as 716 depicted in the ASCII art picture above. 717 </para> 718 719 <para> 720 For example, HMAC(SHA256) is implemented with hmac.c and 721 sha256_generic.c. The following ASCII art illustrates the 722 implementation: 723 </para> 724 725 <programlisting> 726<![CDATA[ 727kernel crypto API | Caller 728 | 729+-----------+ (1) | 730| | <------------------ some_function 731| ahash | 732| (hmac) | ---+ 733+-----------+ | 734 | (2) 735+-----------+ | 736| | <--+ 737| shash | 738| (sha256) | 739+-----------+ 740]]> 741 </programlisting> 742 743 <para> 744 The following call sequence is applicable when a caller triggers 745 an HMAC operation: 746 </para> 747 748 <orderedlist> 749 <listitem> 750 <para> 751 The AHASH API functions are invoked by the caller. The HMAC 752 implementation performs its operation as needed. 753 </para> 754 755 <para> 756 During initialization of the HMAC cipher, the SHASH cipher type of 757 SHA256 is instantiated. The cipher handle for the SHA256 instance is 758 retained. 759 </para> 760 761 <para> 762 At one time, the HMAC implementation requires a SHA256 operation 763 where the SHA256 cipher handle is used. 764 </para> 765 </listitem> 766 767 <listitem> 768 <para> 769 The HMAC instance now invokes the SHASH API with the SHA256 770 cipher handle to calculate the message digest. 771 </para> 772 </listitem> 773 </orderedlist> 774 </sect2> 775 </sect1> 776 </chapter> 777 778 <chapter id="Development"><title>Developing Cipher Algorithms</title> 779 <sect1><title>Registering And Unregistering Transformation</title> 780 <para> 781 There are three distinct types of registration functions in 782 the Crypto API. One is used to register a generic cryptographic 783 transformation, while the other two are specific to HASH 784 transformations and COMPRESSion. We will discuss the latter 785 two in a separate chapter, here we will only look at the 786 generic ones. 787 </para> 788 789 <para> 790 Before discussing the register functions, the data structure 791 to be filled with each, struct crypto_alg, must be considered 792 -- see below for a description of this data structure. 793 </para> 794 795 <para> 796 The generic registration functions can be found in 797 include/linux/crypto.h and their definition can be seen below. 798 The former function registers a single transformation, while 799 the latter works on an array of transformation descriptions. 800 The latter is useful when registering transformations in bulk, 801 for example when a driver implements multiple transformations. 802 </para> 803 804 <programlisting> 805 int crypto_register_alg(struct crypto_alg *alg); 806 int crypto_register_algs(struct crypto_alg *algs, int count); 807 </programlisting> 808 809 <para> 810 The counterparts to those functions are listed below. 811 </para> 812 813 <programlisting> 814 int crypto_unregister_alg(struct crypto_alg *alg); 815 int crypto_unregister_algs(struct crypto_alg *algs, int count); 816 </programlisting> 817 818 <para> 819 Notice that both registration and unregistration functions 820 do return a value, so make sure to handle errors. A return 821 code of zero implies success. Any return code < 0 implies 822 an error. 823 </para> 824 825 <para> 826 The bulk registration/unregistration functions 827 register/unregister each transformation in the given array of 828 length count. They handle errors as follows: 829 </para> 830 <itemizedlist> 831 <listitem> 832 <para> 833 crypto_register_algs() succeeds if and only if it 834 successfully registers all the given transformations. If an 835 error occurs partway through, then it rolls back successful 836 registrations before returning the error code. Note that if 837 a driver needs to handle registration errors for individual 838 transformations, then it will need to use the non-bulk 839 function crypto_register_alg() instead. 840 </para> 841 </listitem> 842 <listitem> 843 <para> 844 crypto_unregister_algs() tries to unregister all the given 845 transformations, continuing on error. It logs errors and 846 always returns zero. 847 </para> 848 </listitem> 849 </itemizedlist> 850 851 </sect1> 852 853 <sect1><title>Single-Block Symmetric Ciphers [CIPHER]</title> 854 <para> 855 Example of transformations: aes, arc4, ... 856 </para> 857 858 <para> 859 This section describes the simplest of all transformation 860 implementations, that being the CIPHER type used for symmetric 861 ciphers. The CIPHER type is used for transformations which 862 operate on exactly one block at a time and there are no 863 dependencies between blocks at all. 864 </para> 865 866 <sect2><title>Registration specifics</title> 867 <para> 868 The registration of [CIPHER] algorithm is specific in that 869 struct crypto_alg field .cra_type is empty. The .cra_u.cipher 870 has to be filled in with proper callbacks to implement this 871 transformation. 872 </para> 873 874 <para> 875 See struct cipher_alg below. 876 </para> 877 </sect2> 878 879 <sect2><title>Cipher Definition With struct cipher_alg</title> 880 <para> 881 Struct cipher_alg defines a single block cipher. 882 </para> 883 884 <para> 885 Here are schematics of how these functions are called when 886 operated from other part of the kernel. Note that the 887 .cia_setkey() call might happen before or after any of these 888 schematics happen, but must not happen during any of these 889 are in-flight. 890 </para> 891 892 <para> 893 <programlisting> 894 KEY ---. PLAINTEXT ---. 895 v v 896 .cia_setkey() -> .cia_encrypt() 897 | 898 '-----> CIPHERTEXT 899 </programlisting> 900 </para> 901 902 <para> 903 Please note that a pattern where .cia_setkey() is called 904 multiple times is also valid: 905 </para> 906 907 <para> 908 <programlisting> 909 910 KEY1 --. PLAINTEXT1 --. KEY2 --. PLAINTEXT2 --. 911 v v v v 912 .cia_setkey() -> .cia_encrypt() -> .cia_setkey() -> .cia_encrypt() 913 | | 914 '---> CIPHERTEXT1 '---> CIPHERTEXT2 915 </programlisting> 916 </para> 917 918 </sect2> 919 </sect1> 920 921 <sect1><title>Multi-Block Ciphers</title> 922 <para> 923 Example of transformations: cbc(aes), ecb(arc4), ... 924 </para> 925 926 <para> 927 This section describes the multi-block cipher transformation 928 implementations. The multi-block ciphers are 929 used for transformations which operate on scatterlists of 930 data supplied to the transformation functions. They output 931 the result into a scatterlist of data as well. 932 </para> 933 934 <sect2><title>Registration Specifics</title> 935 936 <para> 937 The registration of multi-block cipher algorithms 938 is one of the most standard procedures throughout the crypto API. 939 </para> 940 941 <para> 942 Note, if a cipher implementation requires a proper alignment 943 of data, the caller should use the functions of 944 crypto_skcipher_alignmask() to identify a memory alignment mask. 945 The kernel crypto API is able to process requests that are unaligned. 946 This implies, however, additional overhead as the kernel 947 crypto API needs to perform the realignment of the data which 948 may imply moving of data. 949 </para> 950 </sect2> 951 952 <sect2><title>Cipher Definition With struct blkcipher_alg and ablkcipher_alg</title> 953 <para> 954 Struct blkcipher_alg defines a synchronous block cipher whereas 955 struct ablkcipher_alg defines an asynchronous block cipher. 956 </para> 957 958 <para> 959 Please refer to the single block cipher description for schematics 960 of the block cipher usage. 961 </para> 962 </sect2> 963 964 <sect2><title>Specifics Of Asynchronous Multi-Block Cipher</title> 965 <para> 966 There are a couple of specifics to the asynchronous interface. 967 </para> 968 969 <para> 970 First of all, some of the drivers will want to use the 971 Generic ScatterWalk in case the hardware needs to be fed 972 separate chunks of the scatterlist which contains the 973 plaintext and will contain the ciphertext. Please refer 974 to the ScatterWalk interface offered by the Linux kernel 975 scatter / gather list implementation. 976 </para> 977 </sect2> 978 </sect1> 979 980 <sect1><title>Hashing [HASH]</title> 981 982 <para> 983 Example of transformations: crc32, md5, sha1, sha256,... 984 </para> 985 986 <sect2><title>Registering And Unregistering The Transformation</title> 987 988 <para> 989 There are multiple ways to register a HASH transformation, 990 depending on whether the transformation is synchronous [SHASH] 991 or asynchronous [AHASH] and the amount of HASH transformations 992 we are registering. You can find the prototypes defined in 993 include/crypto/internal/hash.h: 994 </para> 995 996 <programlisting> 997 int crypto_register_ahash(struct ahash_alg *alg); 998 999 int crypto_register_shash(struct shash_alg *alg); 1000 int crypto_register_shashes(struct shash_alg *algs, int count); 1001 </programlisting> 1002 1003 <para> 1004 The respective counterparts for unregistering the HASH 1005 transformation are as follows: 1006 </para> 1007 1008 <programlisting> 1009 int crypto_unregister_ahash(struct ahash_alg *alg); 1010 1011 int crypto_unregister_shash(struct shash_alg *alg); 1012 int crypto_unregister_shashes(struct shash_alg *algs, int count); 1013 </programlisting> 1014 </sect2> 1015 1016 <sect2><title>Cipher Definition With struct shash_alg and ahash_alg</title> 1017 <para> 1018 Here are schematics of how these functions are called when 1019 operated from other part of the kernel. Note that the .setkey() 1020 call might happen before or after any of these schematics happen, 1021 but must not happen during any of these are in-flight. Please note 1022 that calling .init() followed immediately by .finish() is also a 1023 perfectly valid transformation. 1024 </para> 1025 1026 <programlisting> 1027 I) DATA -----------. 1028 v 1029 .init() -> .update() -> .final() ! .update() might not be called 1030 ^ | | at all in this scenario. 1031 '----' '---> HASH 1032 1033 II) DATA -----------.-----------. 1034 v v 1035 .init() -> .update() -> .finup() ! .update() may not be called 1036 ^ | | at all in this scenario. 1037 '----' '---> HASH 1038 1039 III) DATA -----------. 1040 v 1041 .digest() ! The entire process is handled 1042 | by the .digest() call. 1043 '---------------> HASH 1044 </programlisting> 1045 1046 <para> 1047 Here is a schematic of how the .export()/.import() functions are 1048 called when used from another part of the kernel. 1049 </para> 1050 1051 <programlisting> 1052 KEY--. DATA--. 1053 v v ! .update() may not be called 1054 .setkey() -> .init() -> .update() -> .export() at all in this scenario. 1055 ^ | | 1056 '-----' '--> PARTIAL_HASH 1057 1058 ----------- other transformations happen here ----------- 1059 1060 PARTIAL_HASH--. DATA1--. 1061 v v 1062 .import -> .update() -> .final() ! .update() may not be called 1063 ^ | | at all in this scenario. 1064 '----' '--> HASH1 1065 1066 PARTIAL_HASH--. DATA2-. 1067 v v 1068 .import -> .finup() 1069 | 1070 '---------------> HASH2 1071 </programlisting> 1072 </sect2> 1073 1074 <sect2><title>Specifics Of Asynchronous HASH Transformation</title> 1075 <para> 1076 Some of the drivers will want to use the Generic ScatterWalk 1077 in case the implementation needs to be fed separate chunks of the 1078 scatterlist which contains the input data. The buffer containing 1079 the resulting hash will always be properly aligned to 1080 .cra_alignmask so there is no need to worry about this. 1081 </para> 1082 </sect2> 1083 </sect1> 1084 </chapter> 1085 1086 <chapter id="User"><title>User Space Interface</title> 1087 <sect1><title>Introduction</title> 1088 <para> 1089 The concepts of the kernel crypto API visible to kernel space is fully 1090 applicable to the user space interface as well. Therefore, the kernel 1091 crypto API high level discussion for the in-kernel use cases applies 1092 here as well. 1093 </para> 1094 1095 <para> 1096 The major difference, however, is that user space can only act as a 1097 consumer and never as a provider of a transformation or cipher algorithm. 1098 </para> 1099 1100 <para> 1101 The following covers the user space interface exported by the kernel 1102 crypto API. A working example of this description is libkcapi that 1103 can be obtained from [1]. That library can be used by user space 1104 applications that require cryptographic services from the kernel. 1105 </para> 1106 1107 <para> 1108 Some details of the in-kernel kernel crypto API aspects do not 1109 apply to user space, however. This includes the difference between 1110 synchronous and asynchronous invocations. The user space API call 1111 is fully synchronous. 1112 </para> 1113 1114 <para> 1115 [1] <ulink url="http://www.chronox.de/libkcapi.html">http://www.chronox.de/libkcapi.html</ulink> 1116 </para> 1117 1118 </sect1> 1119 1120 <sect1><title>User Space API General Remarks</title> 1121 <para> 1122 The kernel crypto API is accessible from user space. Currently, 1123 the following ciphers are accessible: 1124 </para> 1125 1126 <itemizedlist> 1127 <listitem> 1128 <para>Message digest including keyed message digest (HMAC, CMAC)</para> 1129 </listitem> 1130 1131 <listitem> 1132 <para>Symmetric ciphers</para> 1133 </listitem> 1134 1135 <listitem> 1136 <para>AEAD ciphers</para> 1137 </listitem> 1138 1139 <listitem> 1140 <para>Random Number Generators</para> 1141 </listitem> 1142 </itemizedlist> 1143 1144 <para> 1145 The interface is provided via socket type using the type AF_ALG. 1146 In addition, the setsockopt option type is SOL_ALG. In case the 1147 user space header files do not export these flags yet, use the 1148 following macros: 1149 </para> 1150 1151 <programlisting> 1152#ifndef AF_ALG 1153#define AF_ALG 38 1154#endif 1155#ifndef SOL_ALG 1156#define SOL_ALG 279 1157#endif 1158 </programlisting> 1159 1160 <para> 1161 A cipher is accessed with the same name as done for the in-kernel 1162 API calls. This includes the generic vs. unique naming schema for 1163 ciphers as well as the enforcement of priorities for generic names. 1164 </para> 1165 1166 <para> 1167 To interact with the kernel crypto API, a socket must be 1168 created by the user space application. User space invokes the cipher 1169 operation with the send()/write() system call family. The result of the 1170 cipher operation is obtained with the read()/recv() system call family. 1171 </para> 1172 1173 <para> 1174 The following API calls assume that the socket descriptor 1175 is already opened by the user space application and discusses only 1176 the kernel crypto API specific invocations. 1177 </para> 1178 1179 <para> 1180 To initialize the socket interface, the following sequence has to 1181 be performed by the consumer: 1182 </para> 1183 1184 <orderedlist> 1185 <listitem> 1186 <para> 1187 Create a socket of type AF_ALG with the struct sockaddr_alg 1188 parameter specified below for the different cipher types. 1189 </para> 1190 </listitem> 1191 1192 <listitem> 1193 <para> 1194 Invoke bind with the socket descriptor 1195 </para> 1196 </listitem> 1197 1198 <listitem> 1199 <para> 1200 Invoke accept with the socket descriptor. The accept system call 1201 returns a new file descriptor that is to be used to interact with 1202 the particular cipher instance. When invoking send/write or recv/read 1203 system calls to send data to the kernel or obtain data from the 1204 kernel, the file descriptor returned by accept must be used. 1205 </para> 1206 </listitem> 1207 </orderedlist> 1208 </sect1> 1209 1210 <sect1><title>In-place Cipher operation</title> 1211 <para> 1212 Just like the in-kernel operation of the kernel crypto API, the user 1213 space interface allows the cipher operation in-place. That means that 1214 the input buffer used for the send/write system call and the output 1215 buffer used by the read/recv system call may be one and the same. 1216 This is of particular interest for symmetric cipher operations where a 1217 copying of the output data to its final destination can be avoided. 1218 </para> 1219 1220 <para> 1221 If a consumer on the other hand wants to maintain the plaintext and 1222 the ciphertext in different memory locations, all a consumer needs 1223 to do is to provide different memory pointers for the encryption and 1224 decryption operation. 1225 </para> 1226 </sect1> 1227 1228 <sect1><title>Message Digest API</title> 1229 <para> 1230 The message digest type to be used for the cipher operation is 1231 selected when invoking the bind syscall. bind requires the caller 1232 to provide a filled struct sockaddr data structure. This data 1233 structure must be filled as follows: 1234 </para> 1235 1236 <programlisting> 1237struct sockaddr_alg sa = { 1238 .salg_family = AF_ALG, 1239 .salg_type = "hash", /* this selects the hash logic in the kernel */ 1240 .salg_name = "sha1" /* this is the cipher name */ 1241}; 1242 </programlisting> 1243 1244 <para> 1245 The salg_type value "hash" applies to message digests and keyed 1246 message digests. Though, a keyed message digest is referenced by 1247 the appropriate salg_name. Please see below for the setsockopt 1248 interface that explains how the key can be set for a keyed message 1249 digest. 1250 </para> 1251 1252 <para> 1253 Using the send() system call, the application provides the data that 1254 should be processed with the message digest. The send system call 1255 allows the following flags to be specified: 1256 </para> 1257 1258 <itemizedlist> 1259 <listitem> 1260 <para> 1261 MSG_MORE: If this flag is set, the send system call acts like a 1262 message digest update function where the final hash is not 1263 yet calculated. If the flag is not set, the send system call 1264 calculates the final message digest immediately. 1265 </para> 1266 </listitem> 1267 </itemizedlist> 1268 1269 <para> 1270 With the recv() system call, the application can read the message 1271 digest from the kernel crypto API. If the buffer is too small for the 1272 message digest, the flag MSG_TRUNC is set by the kernel. 1273 </para> 1274 1275 <para> 1276 In order to set a message digest key, the calling application must use 1277 the setsockopt() option of ALG_SET_KEY. If the key is not set the HMAC 1278 operation is performed without the initial HMAC state change caused by 1279 the key. 1280 </para> 1281 </sect1> 1282 1283 <sect1><title>Symmetric Cipher API</title> 1284 <para> 1285 The operation is very similar to the message digest discussion. 1286 During initialization, the struct sockaddr data structure must be 1287 filled as follows: 1288 </para> 1289 1290 <programlisting> 1291struct sockaddr_alg sa = { 1292 .salg_family = AF_ALG, 1293 .salg_type = "skcipher", /* this selects the symmetric cipher */ 1294 .salg_name = "cbc(aes)" /* this is the cipher name */ 1295}; 1296 </programlisting> 1297 1298 <para> 1299 Before data can be sent to the kernel using the write/send system 1300 call family, the consumer must set the key. The key setting is 1301 described with the setsockopt invocation below. 1302 </para> 1303 1304 <para> 1305 Using the sendmsg() system call, the application provides the data that should be processed for encryption or decryption. In addition, the IV is 1306 specified with the data structure provided by the sendmsg() system call. 1307 </para> 1308 1309 <para> 1310 The sendmsg system call parameter of struct msghdr is embedded into the 1311 struct cmsghdr data structure. See recv(2) and cmsg(3) for more 1312 information on how the cmsghdr data structure is used together with the 1313 send/recv system call family. That cmsghdr data structure holds the 1314 following information specified with a separate header instances: 1315 </para> 1316 1317 <itemizedlist> 1318 <listitem> 1319 <para> 1320 specification of the cipher operation type with one of these flags: 1321 </para> 1322 <itemizedlist> 1323 <listitem> 1324 <para>ALG_OP_ENCRYPT - encryption of data</para> 1325 </listitem> 1326 <listitem> 1327 <para>ALG_OP_DECRYPT - decryption of data</para> 1328 </listitem> 1329 </itemizedlist> 1330 </listitem> 1331 1332 <listitem> 1333 <para> 1334 specification of the IV information marked with the flag ALG_SET_IV 1335 </para> 1336 </listitem> 1337 </itemizedlist> 1338 1339 <para> 1340 The send system call family allows the following flag to be specified: 1341 </para> 1342 1343 <itemizedlist> 1344 <listitem> 1345 <para> 1346 MSG_MORE: If this flag is set, the send system call acts like a 1347 cipher update function where more input data is expected 1348 with a subsequent invocation of the send system call. 1349 </para> 1350 </listitem> 1351 </itemizedlist> 1352 1353 <para> 1354 Note: The kernel reports -EINVAL for any unexpected data. The caller 1355 must make sure that all data matches the constraints given in 1356 /proc/crypto for the selected cipher. 1357 </para> 1358 1359 <para> 1360 With the recv() system call, the application can read the result of 1361 the cipher operation from the kernel crypto API. The output buffer 1362 must be at least as large as to hold all blocks of the encrypted or 1363 decrypted data. If the output data size is smaller, only as many 1364 blocks are returned that fit into that output buffer size. 1365 </para> 1366 </sect1> 1367 1368 <sect1><title>AEAD Cipher API</title> 1369 <para> 1370 The operation is very similar to the symmetric cipher discussion. 1371 During initialization, the struct sockaddr data structure must be 1372 filled as follows: 1373 </para> 1374 1375 <programlisting> 1376struct sockaddr_alg sa = { 1377 .salg_family = AF_ALG, 1378 .salg_type = "aead", /* this selects the symmetric cipher */ 1379 .salg_name = "gcm(aes)" /* this is the cipher name */ 1380}; 1381 </programlisting> 1382 1383 <para> 1384 Before data can be sent to the kernel using the write/send system 1385 call family, the consumer must set the key. The key setting is 1386 described with the setsockopt invocation below. 1387 </para> 1388 1389 <para> 1390 In addition, before data can be sent to the kernel using the 1391 write/send system call family, the consumer must set the authentication 1392 tag size. To set the authentication tag size, the caller must use the 1393 setsockopt invocation described below. 1394 </para> 1395 1396 <para> 1397 Using the sendmsg() system call, the application provides the data that should be processed for encryption or decryption. In addition, the IV is 1398 specified with the data structure provided by the sendmsg() system call. 1399 </para> 1400 1401 <para> 1402 The sendmsg system call parameter of struct msghdr is embedded into the 1403 struct cmsghdr data structure. See recv(2) and cmsg(3) for more 1404 information on how the cmsghdr data structure is used together with the 1405 send/recv system call family. That cmsghdr data structure holds the 1406 following information specified with a separate header instances: 1407 </para> 1408 1409 <itemizedlist> 1410 <listitem> 1411 <para> 1412 specification of the cipher operation type with one of these flags: 1413 </para> 1414 <itemizedlist> 1415 <listitem> 1416 <para>ALG_OP_ENCRYPT - encryption of data</para> 1417 </listitem> 1418 <listitem> 1419 <para>ALG_OP_DECRYPT - decryption of data</para> 1420 </listitem> 1421 </itemizedlist> 1422 </listitem> 1423 1424 <listitem> 1425 <para> 1426 specification of the IV information marked with the flag ALG_SET_IV 1427 </para> 1428 </listitem> 1429 1430 <listitem> 1431 <para> 1432 specification of the associated authentication data (AAD) with the 1433 flag ALG_SET_AEAD_ASSOCLEN. The AAD is sent to the kernel together 1434 with the plaintext / ciphertext. See below for the memory structure. 1435 </para> 1436 </listitem> 1437 </itemizedlist> 1438 1439 <para> 1440 The send system call family allows the following flag to be specified: 1441 </para> 1442 1443 <itemizedlist> 1444 <listitem> 1445 <para> 1446 MSG_MORE: If this flag is set, the send system call acts like a 1447 cipher update function where more input data is expected 1448 with a subsequent invocation of the send system call. 1449 </para> 1450 </listitem> 1451 </itemizedlist> 1452 1453 <para> 1454 Note: The kernel reports -EINVAL for any unexpected data. The caller 1455 must make sure that all data matches the constraints given in 1456 /proc/crypto for the selected cipher. 1457 </para> 1458 1459 <para> 1460 With the recv() system call, the application can read the result of 1461 the cipher operation from the kernel crypto API. The output buffer 1462 must be at least as large as defined with the memory structure below. 1463 If the output data size is smaller, the cipher operation is not performed. 1464 </para> 1465 1466 <para> 1467 The authenticated decryption operation may indicate an integrity error. 1468 Such breach in integrity is marked with the -EBADMSG error code. 1469 </para> 1470 1471 <sect2><title>AEAD Memory Structure</title> 1472 <para> 1473 The AEAD cipher operates with the following information that 1474 is communicated between user and kernel space as one data stream: 1475 </para> 1476 1477 <itemizedlist> 1478 <listitem> 1479 <para>plaintext or ciphertext</para> 1480 </listitem> 1481 1482 <listitem> 1483 <para>associated authentication data (AAD)</para> 1484 </listitem> 1485 1486 <listitem> 1487 <para>authentication tag</para> 1488 </listitem> 1489 </itemizedlist> 1490 1491 <para> 1492 The sizes of the AAD and the authentication tag are provided with 1493 the sendmsg and setsockopt calls (see there). As the kernel knows 1494 the size of the entire data stream, the kernel is now able to 1495 calculate the right offsets of the data components in the data 1496 stream. 1497 </para> 1498 1499 <para> 1500 The user space caller must arrange the aforementioned information 1501 in the following order: 1502 </para> 1503 1504 <itemizedlist> 1505 <listitem> 1506 <para> 1507 AEAD encryption input: AAD || plaintext 1508 </para> 1509 </listitem> 1510 1511 <listitem> 1512 <para> 1513 AEAD decryption input: AAD || ciphertext || authentication tag 1514 </para> 1515 </listitem> 1516 </itemizedlist> 1517 1518 <para> 1519 The output buffer the user space caller provides must be at least as 1520 large to hold the following data: 1521 </para> 1522 1523 <itemizedlist> 1524 <listitem> 1525 <para> 1526 AEAD encryption output: ciphertext || authentication tag 1527 </para> 1528 </listitem> 1529 1530 <listitem> 1531 <para> 1532 AEAD decryption output: plaintext 1533 </para> 1534 </listitem> 1535 </itemizedlist> 1536 </sect2> 1537 </sect1> 1538 1539 <sect1><title>Random Number Generator API</title> 1540 <para> 1541 Again, the operation is very similar to the other APIs. 1542 During initialization, the struct sockaddr data structure must be 1543 filled as follows: 1544 </para> 1545 1546 <programlisting> 1547struct sockaddr_alg sa = { 1548 .salg_family = AF_ALG, 1549 .salg_type = "rng", /* this selects the symmetric cipher */ 1550 .salg_name = "drbg_nopr_sha256" /* this is the cipher name */ 1551}; 1552 </programlisting> 1553 1554 <para> 1555 Depending on the RNG type, the RNG must be seeded. The seed is provided 1556 using the setsockopt interface to set the key. For example, the 1557 ansi_cprng requires a seed. The DRBGs do not require a seed, but 1558 may be seeded. 1559 </para> 1560 1561 <para> 1562 Using the read()/recvmsg() system calls, random numbers can be obtained. 1563 The kernel generates at most 128 bytes in one call. If user space 1564 requires more data, multiple calls to read()/recvmsg() must be made. 1565 </para> 1566 1567 <para> 1568 WARNING: The user space caller may invoke the initially mentioned 1569 accept system call multiple times. In this case, the returned file 1570 descriptors have the same state. 1571 </para> 1572 1573 </sect1> 1574 1575 <sect1><title>Zero-Copy Interface</title> 1576 <para> 1577 In addition to the send/write/read/recv system call family, the AF_ALG 1578 interface can be accessed with the zero-copy interface of splice/vmsplice. 1579 As the name indicates, the kernel tries to avoid a copy operation into 1580 kernel space. 1581 </para> 1582 1583 <para> 1584 The zero-copy operation requires data to be aligned at the page boundary. 1585 Non-aligned data can be used as well, but may require more operations of 1586 the kernel which would defeat the speed gains obtained from the zero-copy 1587 interface. 1588 </para> 1589 1590 <para> 1591 The system-interent limit for the size of one zero-copy operation is 1592 16 pages. If more data is to be sent to AF_ALG, user space must slice 1593 the input into segments with a maximum size of 16 pages. 1594 </para> 1595 1596 <para> 1597 Zero-copy can be used with the following code example (a complete working 1598 example is provided with libkcapi): 1599 </para> 1600 1601 <programlisting> 1602int pipes[2]; 1603 1604pipe(pipes); 1605/* input data in iov */ 1606vmsplice(pipes[1], iov, iovlen, SPLICE_F_GIFT); 1607/* opfd is the file descriptor returned from accept() system call */ 1608splice(pipes[0], NULL, opfd, NULL, ret, 0); 1609read(opfd, out, outlen); 1610 </programlisting> 1611 1612 </sect1> 1613 1614 <sect1><title>Setsockopt Interface</title> 1615 <para> 1616 In addition to the read/recv and send/write system call handling 1617 to send and retrieve data subject to the cipher operation, a consumer 1618 also needs to set the additional information for the cipher operation. 1619 This additional information is set using the setsockopt system call 1620 that must be invoked with the file descriptor of the open cipher 1621 (i.e. the file descriptor returned by the accept system call). 1622 </para> 1623 1624 <para> 1625 Each setsockopt invocation must use the level SOL_ALG. 1626 </para> 1627 1628 <para> 1629 The setsockopt interface allows setting the following data using 1630 the mentioned optname: 1631 </para> 1632 1633 <itemizedlist> 1634 <listitem> 1635 <para> 1636 ALG_SET_KEY -- Setting the key. Key setting is applicable to: 1637 </para> 1638 <itemizedlist> 1639 <listitem> 1640 <para>the skcipher cipher type (symmetric ciphers)</para> 1641 </listitem> 1642 <listitem> 1643 <para>the hash cipher type (keyed message digests)</para> 1644 </listitem> 1645 <listitem> 1646 <para>the AEAD cipher type</para> 1647 </listitem> 1648 <listitem> 1649 <para>the RNG cipher type to provide the seed</para> 1650 </listitem> 1651 </itemizedlist> 1652 </listitem> 1653 1654 <listitem> 1655 <para> 1656 ALG_SET_AEAD_AUTHSIZE -- Setting the authentication tag size 1657 for AEAD ciphers. For a encryption operation, the authentication 1658 tag of the given size will be generated. For a decryption operation, 1659 the provided ciphertext is assumed to contain an authentication tag 1660 of the given size (see section about AEAD memory layout below). 1661 </para> 1662 </listitem> 1663 </itemizedlist> 1664 1665 </sect1> 1666 1667 <sect1><title>User space API example</title> 1668 <para> 1669 Please see [1] for libkcapi which provides an easy-to-use wrapper 1670 around the aforementioned Netlink kernel interface. [1] also contains 1671 a test application that invokes all libkcapi API calls. 1672 </para> 1673 1674 <para> 1675 [1] <ulink url="http://www.chronox.de/libkcapi.html">http://www.chronox.de/libkcapi.html</ulink> 1676 </para> 1677 1678 </sect1> 1679 1680 </chapter> 1681 1682 <chapter id="API"><title>Programming Interface</title> 1683 <para> 1684 Please note that the kernel crypto API contains the AEAD givcrypt 1685 API (crypto_aead_giv* and aead_givcrypt_* function calls in 1686 include/crypto/aead.h). This API is obsolete and will be removed 1687 in the future. To obtain the functionality of an AEAD cipher with 1688 internal IV generation, use the IV generator as a regular cipher. 1689 For example, rfc4106(gcm(aes)) is the AEAD cipher with external 1690 IV generation and seqniv(rfc4106(gcm(aes))) implies that the kernel 1691 crypto API generates the IV. Different IV generators are available. 1692 </para> 1693 <sect1><title>Block Cipher Context Data Structures</title> 1694!Pinclude/linux/crypto.h Block Cipher Context Data Structures 1695!Finclude/crypto/aead.h aead_request 1696 </sect1> 1697 <sect1><title>Block Cipher Algorithm Definitions</title> 1698!Pinclude/linux/crypto.h Block Cipher Algorithm Definitions 1699!Finclude/linux/crypto.h crypto_alg 1700!Finclude/linux/crypto.h ablkcipher_alg 1701!Finclude/crypto/aead.h aead_alg 1702!Finclude/linux/crypto.h blkcipher_alg 1703!Finclude/linux/crypto.h cipher_alg 1704!Finclude/crypto/rng.h rng_alg 1705 </sect1> 1706 <sect1><title>Symmetric Key Cipher API</title> 1707!Pinclude/crypto/skcipher.h Symmetric Key Cipher API 1708!Finclude/crypto/skcipher.h crypto_alloc_skcipher 1709!Finclude/crypto/skcipher.h crypto_free_skcipher 1710!Finclude/crypto/skcipher.h crypto_has_skcipher 1711!Finclude/crypto/skcipher.h crypto_skcipher_ivsize 1712!Finclude/crypto/skcipher.h crypto_skcipher_blocksize 1713!Finclude/crypto/skcipher.h crypto_skcipher_setkey 1714!Finclude/crypto/skcipher.h crypto_skcipher_reqtfm 1715!Finclude/crypto/skcipher.h crypto_skcipher_encrypt 1716!Finclude/crypto/skcipher.h crypto_skcipher_decrypt 1717 </sect1> 1718 <sect1><title>Symmetric Key Cipher Request Handle</title> 1719!Pinclude/crypto/skcipher.h Symmetric Key Cipher Request Handle 1720!Finclude/crypto/skcipher.h crypto_skcipher_reqsize 1721!Finclude/crypto/skcipher.h skcipher_request_set_tfm 1722!Finclude/crypto/skcipher.h skcipher_request_alloc 1723!Finclude/crypto/skcipher.h skcipher_request_free 1724!Finclude/crypto/skcipher.h skcipher_request_set_callback 1725!Finclude/crypto/skcipher.h skcipher_request_set_crypt 1726 </sect1> 1727 <sect1><title>Asynchronous Block Cipher API - Deprecated</title> 1728!Pinclude/linux/crypto.h Asynchronous Block Cipher API 1729!Finclude/linux/crypto.h crypto_alloc_ablkcipher 1730!Finclude/linux/crypto.h crypto_free_ablkcipher 1731!Finclude/linux/crypto.h crypto_has_ablkcipher 1732!Finclude/linux/crypto.h crypto_ablkcipher_ivsize 1733!Finclude/linux/crypto.h crypto_ablkcipher_blocksize 1734!Finclude/linux/crypto.h crypto_ablkcipher_setkey 1735!Finclude/linux/crypto.h crypto_ablkcipher_reqtfm 1736!Finclude/linux/crypto.h crypto_ablkcipher_encrypt 1737!Finclude/linux/crypto.h crypto_ablkcipher_decrypt 1738 </sect1> 1739 <sect1><title>Asynchronous Cipher Request Handle - Deprecated</title> 1740!Pinclude/linux/crypto.h Asynchronous Cipher Request Handle 1741!Finclude/linux/crypto.h crypto_ablkcipher_reqsize 1742!Finclude/linux/crypto.h ablkcipher_request_set_tfm 1743!Finclude/linux/crypto.h ablkcipher_request_alloc 1744!Finclude/linux/crypto.h ablkcipher_request_free 1745!Finclude/linux/crypto.h ablkcipher_request_set_callback 1746!Finclude/linux/crypto.h ablkcipher_request_set_crypt 1747 </sect1> 1748 <sect1><title>Authenticated Encryption With Associated Data (AEAD) Cipher API</title> 1749!Pinclude/crypto/aead.h Authenticated Encryption With Associated Data (AEAD) Cipher API 1750!Finclude/crypto/aead.h crypto_alloc_aead 1751!Finclude/crypto/aead.h crypto_free_aead 1752!Finclude/crypto/aead.h crypto_aead_ivsize 1753!Finclude/crypto/aead.h crypto_aead_authsize 1754!Finclude/crypto/aead.h crypto_aead_blocksize 1755!Finclude/crypto/aead.h crypto_aead_setkey 1756!Finclude/crypto/aead.h crypto_aead_setauthsize 1757!Finclude/crypto/aead.h crypto_aead_encrypt 1758!Finclude/crypto/aead.h crypto_aead_decrypt 1759 </sect1> 1760 <sect1><title>Asynchronous AEAD Request Handle</title> 1761!Pinclude/crypto/aead.h Asynchronous AEAD Request Handle 1762!Finclude/crypto/aead.h crypto_aead_reqsize 1763!Finclude/crypto/aead.h aead_request_set_tfm 1764!Finclude/crypto/aead.h aead_request_alloc 1765!Finclude/crypto/aead.h aead_request_free 1766!Finclude/crypto/aead.h aead_request_set_callback 1767!Finclude/crypto/aead.h aead_request_set_crypt 1768!Finclude/crypto/aead.h aead_request_set_ad 1769 </sect1> 1770 <sect1><title>Synchronous Block Cipher API - Deprecated</title> 1771!Pinclude/linux/crypto.h Synchronous Block Cipher API 1772!Finclude/linux/crypto.h crypto_alloc_blkcipher 1773!Finclude/linux/crypto.h crypto_free_blkcipher 1774!Finclude/linux/crypto.h crypto_has_blkcipher 1775!Finclude/linux/crypto.h crypto_blkcipher_name 1776!Finclude/linux/crypto.h crypto_blkcipher_ivsize 1777!Finclude/linux/crypto.h crypto_blkcipher_blocksize 1778!Finclude/linux/crypto.h crypto_blkcipher_setkey 1779!Finclude/linux/crypto.h crypto_blkcipher_encrypt 1780!Finclude/linux/crypto.h crypto_blkcipher_encrypt_iv 1781!Finclude/linux/crypto.h crypto_blkcipher_decrypt 1782!Finclude/linux/crypto.h crypto_blkcipher_decrypt_iv 1783!Finclude/linux/crypto.h crypto_blkcipher_set_iv 1784!Finclude/linux/crypto.h crypto_blkcipher_get_iv 1785 </sect1> 1786 <sect1><title>Single Block Cipher API</title> 1787!Pinclude/linux/crypto.h Single Block Cipher API 1788!Finclude/linux/crypto.h crypto_alloc_cipher 1789!Finclude/linux/crypto.h crypto_free_cipher 1790!Finclude/linux/crypto.h crypto_has_cipher 1791!Finclude/linux/crypto.h crypto_cipher_blocksize 1792!Finclude/linux/crypto.h crypto_cipher_setkey 1793!Finclude/linux/crypto.h crypto_cipher_encrypt_one 1794!Finclude/linux/crypto.h crypto_cipher_decrypt_one 1795 </sect1> 1796 <sect1><title>Message Digest Algorithm Definitions</title> 1797!Pinclude/crypto/hash.h Message Digest Algorithm Definitions 1798!Finclude/crypto/hash.h hash_alg_common 1799!Finclude/crypto/hash.h ahash_alg 1800!Finclude/crypto/hash.h shash_alg 1801 </sect1> 1802 <sect1><title>Asynchronous Message Digest API</title> 1803!Pinclude/crypto/hash.h Asynchronous Message Digest API 1804!Finclude/crypto/hash.h crypto_alloc_ahash 1805!Finclude/crypto/hash.h crypto_free_ahash 1806!Finclude/crypto/hash.h crypto_ahash_init 1807!Finclude/crypto/hash.h crypto_ahash_digestsize 1808!Finclude/crypto/hash.h crypto_ahash_reqtfm 1809!Finclude/crypto/hash.h crypto_ahash_reqsize 1810!Finclude/crypto/hash.h crypto_ahash_setkey 1811!Finclude/crypto/hash.h crypto_ahash_finup 1812!Finclude/crypto/hash.h crypto_ahash_final 1813!Finclude/crypto/hash.h crypto_ahash_digest 1814!Finclude/crypto/hash.h crypto_ahash_export 1815!Finclude/crypto/hash.h crypto_ahash_import 1816 </sect1> 1817 <sect1><title>Asynchronous Hash Request Handle</title> 1818!Pinclude/crypto/hash.h Asynchronous Hash Request Handle 1819!Finclude/crypto/hash.h ahash_request_set_tfm 1820!Finclude/crypto/hash.h ahash_request_alloc 1821!Finclude/crypto/hash.h ahash_request_free 1822!Finclude/crypto/hash.h ahash_request_set_callback 1823!Finclude/crypto/hash.h ahash_request_set_crypt 1824 </sect1> 1825 <sect1><title>Synchronous Message Digest API</title> 1826!Pinclude/crypto/hash.h Synchronous Message Digest API 1827!Finclude/crypto/hash.h crypto_alloc_shash 1828!Finclude/crypto/hash.h crypto_free_shash 1829!Finclude/crypto/hash.h crypto_shash_blocksize 1830!Finclude/crypto/hash.h crypto_shash_digestsize 1831!Finclude/crypto/hash.h crypto_shash_descsize 1832!Finclude/crypto/hash.h crypto_shash_setkey 1833!Finclude/crypto/hash.h crypto_shash_digest 1834!Finclude/crypto/hash.h crypto_shash_export 1835!Finclude/crypto/hash.h crypto_shash_import 1836!Finclude/crypto/hash.h crypto_shash_init 1837!Finclude/crypto/hash.h crypto_shash_update 1838!Finclude/crypto/hash.h crypto_shash_final 1839!Finclude/crypto/hash.h crypto_shash_finup 1840 </sect1> 1841 <sect1><title>Crypto API Random Number API</title> 1842!Pinclude/crypto/rng.h Random number generator API 1843!Finclude/crypto/rng.h crypto_alloc_rng 1844!Finclude/crypto/rng.h crypto_rng_alg 1845!Finclude/crypto/rng.h crypto_free_rng 1846!Finclude/crypto/rng.h crypto_rng_generate 1847!Finclude/crypto/rng.h crypto_rng_get_bytes 1848!Finclude/crypto/rng.h crypto_rng_reset 1849!Finclude/crypto/rng.h crypto_rng_seedsize 1850!Cinclude/crypto/rng.h 1851 </sect1> 1852 <sect1><title>Asymmetric Cipher API</title> 1853!Pinclude/crypto/akcipher.h Generic Public Key API 1854!Finclude/crypto/akcipher.h akcipher_alg 1855!Finclude/crypto/akcipher.h akcipher_request 1856!Finclude/crypto/akcipher.h crypto_alloc_akcipher 1857!Finclude/crypto/akcipher.h crypto_free_akcipher 1858!Finclude/crypto/akcipher.h crypto_akcipher_set_pub_key 1859!Finclude/crypto/akcipher.h crypto_akcipher_set_priv_key 1860 </sect1> 1861 <sect1><title>Asymmetric Cipher Request Handle</title> 1862!Finclude/crypto/akcipher.h akcipher_request_alloc 1863!Finclude/crypto/akcipher.h akcipher_request_free 1864!Finclude/crypto/akcipher.h akcipher_request_set_callback 1865!Finclude/crypto/akcipher.h akcipher_request_set_crypt 1866!Finclude/crypto/akcipher.h crypto_akcipher_maxsize 1867!Finclude/crypto/akcipher.h crypto_akcipher_encrypt 1868!Finclude/crypto/akcipher.h crypto_akcipher_decrypt 1869!Finclude/crypto/akcipher.h crypto_akcipher_sign 1870!Finclude/crypto/akcipher.h crypto_akcipher_verify 1871 </sect1> 1872 </chapter> 1873 1874 <chapter id="Code"><title>Code Examples</title> 1875 <sect1><title>Code Example For Symmetric Key Cipher Operation</title> 1876 <programlisting> 1877 1878struct tcrypt_result { 1879 struct completion completion; 1880 int err; 1881}; 1882 1883/* tie all data structures together */ 1884struct skcipher_def { 1885 struct scatterlist sg; 1886 struct crypto_skcipher *tfm; 1887 struct skcipher_request *req; 1888 struct tcrypt_result result; 1889}; 1890 1891/* Callback function */ 1892static void test_skcipher_cb(struct crypto_async_request *req, int error) 1893{ 1894 struct tcrypt_result *result = req->data; 1895 1896 if (error == -EINPROGRESS) 1897 return; 1898 result->err = error; 1899 complete(&result->completion); 1900 pr_info("Encryption finished successfully\n"); 1901} 1902 1903/* Perform cipher operation */ 1904static unsigned int test_skcipher_encdec(struct skcipher_def *sk, 1905 int enc) 1906{ 1907 int rc = 0; 1908 1909 if (enc) 1910 rc = crypto_skcipher_encrypt(sk->req); 1911 else 1912 rc = crypto_skcipher_decrypt(sk->req); 1913 1914 switch (rc) { 1915 case 0: 1916 break; 1917 case -EINPROGRESS: 1918 case -EBUSY: 1919 rc = wait_for_completion_interruptible( 1920 &sk->result.completion); 1921 if (!rc && !sk->result.err) { 1922 reinit_completion(&sk->result.completion); 1923 break; 1924 } 1925 default: 1926 pr_info("skcipher encrypt returned with %d result %d\n", 1927 rc, sk->result.err); 1928 break; 1929 } 1930 init_completion(&sk->result.completion); 1931 1932 return rc; 1933} 1934 1935/* Initialize and trigger cipher operation */ 1936static int test_skcipher(void) 1937{ 1938 struct skcipher_def sk; 1939 struct crypto_skcipher *skcipher = NULL; 1940 struct skcipher_request *req = NULL; 1941 char *scratchpad = NULL; 1942 char *ivdata = NULL; 1943 unsigned char key[32]; 1944 int ret = -EFAULT; 1945 1946 skcipher = crypto_alloc_skcipher("cbc-aes-aesni", 0, 0); 1947 if (IS_ERR(skcipher)) { 1948 pr_info("could not allocate skcipher handle\n"); 1949 return PTR_ERR(skcipher); 1950 } 1951 1952 req = skcipher_request_alloc(skcipher, GFP_KERNEL); 1953 if (!req) { 1954 pr_info("could not allocate skcipher request\n"); 1955 ret = -ENOMEM; 1956 goto out; 1957 } 1958 1959 skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG, 1960 test_skcipher_cb, 1961 &sk.result); 1962 1963 /* AES 256 with random key */ 1964 get_random_bytes(&key, 32); 1965 if (crypto_skcipher_setkey(skcipher, key, 32)) { 1966 pr_info("key could not be set\n"); 1967 ret = -EAGAIN; 1968 goto out; 1969 } 1970 1971 /* IV will be random */ 1972 ivdata = kmalloc(16, GFP_KERNEL); 1973 if (!ivdata) { 1974 pr_info("could not allocate ivdata\n"); 1975 goto out; 1976 } 1977 get_random_bytes(ivdata, 16); 1978 1979 /* Input data will be random */ 1980 scratchpad = kmalloc(16, GFP_KERNEL); 1981 if (!scratchpad) { 1982 pr_info("could not allocate scratchpad\n"); 1983 goto out; 1984 } 1985 get_random_bytes(scratchpad, 16); 1986 1987 sk.tfm = skcipher; 1988 sk.req = req; 1989 1990 /* We encrypt one block */ 1991 sg_init_one(&sk.sg, scratchpad, 16); 1992 skcipher_request_set_crypt(req, &sk.sg, &sk.sg, 16, ivdata); 1993 init_completion(&sk.result.completion); 1994 1995 /* encrypt data */ 1996 ret = test_skcipher_encdec(&sk, 1); 1997 if (ret) 1998 goto out; 1999 2000 pr_info("Encryption triggered successfully\n"); 2001 2002out: 2003 if (skcipher) 2004 crypto_free_skcipher(skcipher); 2005 if (req) 2006 skcipher_request_free(req); 2007 if (ivdata) 2008 kfree(ivdata); 2009 if (scratchpad) 2010 kfree(scratchpad); 2011 return ret; 2012} 2013 </programlisting> 2014 </sect1> 2015 2016 <sect1><title>Code Example For Use of Operational State Memory With SHASH</title> 2017 <programlisting> 2018 2019struct sdesc { 2020 struct shash_desc shash; 2021 char ctx[]; 2022}; 2023 2024static struct sdescinit_sdesc(struct crypto_shash *alg) 2025{ 2026 struct sdescsdesc; 2027 int size; 2028 2029 size = sizeof(struct shash_desc) + crypto_shash_descsize(alg); 2030 sdesc = kmalloc(size, GFP_KERNEL); 2031 if (!sdesc) 2032 return ERR_PTR(-ENOMEM); 2033 sdesc->shash.tfm = alg; 2034 sdesc->shash.flags = 0x0; 2035 return sdesc; 2036} 2037 2038static int calc_hash(struct crypto_shashalg, 2039 const unsigned chardata, unsigned int datalen, 2040 unsigned chardigest) { 2041 struct sdescsdesc; 2042 int ret; 2043 2044 sdesc = init_sdesc(alg); 2045 if (IS_ERR(sdesc)) { 2046 pr_info("trusted_key: can't alloc %s\n", hash_alg); 2047 return PTR_ERR(sdesc); 2048 } 2049 2050 ret = crypto_shash_digest(&sdesc->shash, data, datalen, digest); 2051 kfree(sdesc); 2052 return ret; 2053} 2054 </programlisting> 2055 </sect1> 2056 2057 <sect1><title>Code Example For Random Number Generator Usage</title> 2058 <programlisting> 2059 2060static int get_random_numbers(u8 *buf, unsigned int len) 2061{ 2062 struct crypto_rngrng = NULL; 2063 chardrbg = "drbg_nopr_sha256"; /* Hash DRBG with SHA-256, no PR */ 2064 int ret; 2065 2066 if (!buf || !len) { 2067 pr_debug("No output buffer provided\n"); 2068 return -EINVAL; 2069 } 2070 2071 rng = crypto_alloc_rng(drbg, 0, 0); 2072 if (IS_ERR(rng)) { 2073 pr_debug("could not allocate RNG handle for %s\n", drbg); 2074 return -PTR_ERR(rng); 2075 } 2076 2077 ret = crypto_rng_get_bytes(rng, buf, len); 2078 if (ret < 0) 2079 pr_debug("generation of random numbers failed\n"); 2080 else if (ret == 0) 2081 pr_debug("RNG returned no data"); 2082 else 2083 pr_debug("RNG returned %d bytes of data\n", ret); 2084 2085out: 2086 crypto_free_rng(rng); 2087 return ret; 2088} 2089 </programlisting> 2090 </sect1> 2091 </chapter> 2092 </book> 2093